Make the mapping of problem index to problem files less obscure

[gehring]

The mapping of problem index to problem files is not visible without diving into the code. Either we establish an explicit mapping of indices to pddl problem files, e.g., a yaml with a list of pddl file names or a dict of index and file names, or we keep the problem index private and remove it from the public API, e.g., replace PDDLEnv.fix_problem_index(index) with something like PDDLEnv.fix_problem(problem_filepath).

[tomsilver]

Yup agreed. I like fix_problem(problem_filepath) as an option.